The separate physiological functions of cyclic nucleotide-dependent protein kinases will be studied. The effects of hormones on the phosphorylation state of the regulatory subunit of protein kinase in rat heart and liver will be examined. The functional domains of the enzymes will be separated, identified, and characterized. The mechanism of activation of the kinase by cyclic nucleotides will be investigated. Other cyclic nucleotide receptors will be identified, purified, and characterized.